The invention relates generally to concentrates employed in the formation of step-growth polymers, and in particular, to a chain extension concentrate for step-growth polymers.
Many step-growth polymers, including polyesters, polyamides, polycarbonates and polyurethanes are widely used to make plastic products such as films, bottles, sheets and other molded and extruded products. The mechanical and physical properties of these polymers are highly dependent on their molecular weights.
In a life cycle, these materials may experience a synthesis process, followed by an extrusion step and a final processing step which may be another compounding extrusion operation followed by thermoforming, blow molding or fiber spinning or they can be injection molded in the molten state, with all of these steps occurring under high temperature conditions. In addition, in recent years, increased attention has been focused on improved methods of recycling articles made from these polymers, with regarding resource conservation and environmental protection. The processing steps involved in producing and recycling these polymers also involve high temperatures.
In each of these high temperature steps, particularly during the compounding/processing and reclaiming/recycling process some molecular weight degradation in the polymer occurs. This molecular weight degradation may occur via high temperature hydrolysis, alcoholysis or other depolymerisation mechanisms well known for these polycondensates. It is also well known that degradation of molecular weight negatively affects the mechanical, thermal and rheological properties of materials, thus preventing them from being used in demanding applications or from being recycled in large proportions in their original applications. Today recycled or reprocessed polycondensates with deteriorated weight can only be used in very low proportions in demanding applications or in larger proportions in less demanding applications. For instance, due to molecular weight degradation, recycled bottle grade polyethylene terephthalate (PET) is mostly employed exclusively in films and other low end applications. Similarly, recycled polycarbonate from compact disk (CD) scrap, mostly goes to low end applications. For these reasons, the current recycling technologies are limited to a narrow range of applications.
Today, there exists a considerable number of processes which are employed to minimize loss in molecular weight and maintain or even increase the molecular weight of the polycondensates for processing or recycling. Most of these routes employ as main processing equipment either extruder, solid state polycondensation reactor or both in sequence or similar equipment designed for melt or high viscosity material processing. As processing aid in any process, chemical reactants known as “chain extenders” are employed. Chain extenders usually are multi functional molecules which “recouple” polycondensate chains that have depolymerized. These chain extenders were added to the extruder or reactor while processing the polymer. Normally chain extenders possess two or more functional groups which can react with chain fragments, caused by depolymerisation, to bridge and couple them. That process can stop decreasing or even increase molecular weight of polycondensates. There are numerous chain extender types, compositions, polycondensate formulations and processing conditions which will be described.
Di- or polyfunctional epoxides, epoxy resins or other chemicals having two or more epoxy groups are examples of chain extending modifiers which have been used to increase the molecular weight of recycled polymers. These di- or polyfunctional epoxides are made of epichlorohydrin and molecules with two or more terminal hydroxyl groups. Examples of such chain extenders include bis-phenol type epoxy compounds, made of bisphenol-A and epichlorohydrin, novolak type epoxy compounds made of carboxylic acids and epichlorohydrin and glycidyl ethers made of aliphatic alcohols and epichlorohydrin. Additionally, various acrylic copolymers have been used as polymer additives to improve melt strength and melt viscosity of polyesters and polycarbonates. These additives generally include copolymers derived from various epoxy containing compounds and olefins, like ethylene. However, these chain extenders only exhibit moderate success in prohibiting degradation in reprocessed polymers.
Today two main problems persist with the state of the art-solutions. In order to have efficient chain extension at reasonable residence times either in extrusion or solid state reactor systems, most of known chain extenders require the use of pre-dried polycondensate material, operating at vacuum and varying amounts of catalysts and stabilizers to be employed during processing. Without these features the extent of molecular weight increase is limited and the resulting product shows lower molecular weight and less than desired properties.
As the functionality of chain extender increases, so does the number of polycondensate chains that can be coupled onto each chain extender molecule and thus its effectiveness in re-building molecular weight. However it's obvious to see that increasing the functionality of chain extenders also increases degree of branching of the resulting product and the potential onset of gelation. There are negative effects of extensive branching on degree of crystallinity and thus on mechanical properties of semi-crystalline polycondensate, as well as negative implications of the presence of varying amounts of gel in any product. As result of these negative effects there is a limit for the maximum functionality. Effective chain extension currently requires relatively large concentration of lower functionality (<4 functional groups per chain) chain extenders.
The relatively high costs associated with these two limitations of the current art render the re-processing or recycling of these polycondensation uneconomical.
One type of chain extender that has been effective in overcoming the problems encountered by the prior art are those based on epoxy-functionalized styrene acrylic copolymers produced from monomers of at least one epoxy-functional acrylic monomer and at least non-functional styrenic and/or acrylate monomer. These chain extenders also exhibit certain disadvantages when introduced directly into a molding apparatus. The chain extenders are difficult to pelletize or otherwise agglomerate. Furthermore, the epoxy-functionalized styrene acrylic copolymer chain extenders are highly reactive in comparison to prior chain extenders. As a result, with certain applications the epoxy-functional styrene acrylic copolymer chain extenders have a tendency to produce overreaction conditions in the feed or introduction zone of a molding apparatus or extruder. These overreaction conditions are a consequence of the disparity in melting temperature between the epoxy-functional styrene acrylic copolymer chain extenders and the step-growth polymers with which they are employed. The epoxy-functional styrene acrylic copolymer chain extenders have a melting temperature of approximately 50° C., whereas the typical process temperatures for step-growth polymers can range from approximately 240° C. to 300° C. Thus, when the epoxy-functional styrene acrylic copolymer chain extenders are introduced directly to the feed zone of a processing apparatus, the chain extender melts and begins to react with step-growth polymer before proper dispersion and homogenization is achieved. When the epoxy-functional styrene acrylic copolymer chain extenders prematurely react, localized areas of overreaction produce gelation which in turn interferes with proper particle formation. The problem of over reaction is especially pronounced when manufacturing particles having a minimal thickness, such as e.g. fibers or films.
Consequently, there exists a need in the industry for a method and a concentrate composition or masterbatch which can effectively deliver and allow proper homogenization of chain extenders within polymers. Also because of some acrylic epoxy-functionalized chain extenders contain components which may cause cancer.